Molecules that toggle between two distinct forms when exposed to specific external stimuli, where each form exhibits unique physical properties, are promising candidates for fabricating controllable nano-devices.1 Photochromic devices exhibit reversible variations in color when stimulated by light.2 Few photochromic compounds possess the favourable properties displayed by the 1,2-dithienylcyclopentene skeleton, which interconverts between its colorless ring-open and colored ring-closed isomers with a high degree of fatigue resistance and bistability.3 Photochromic compounds have many potential applications including high-density optical information storage systems, photoregulated molecular switches, reversible holographic systems, ophthalmic lenses, actinometry and molecular sensors, photochromic inks, paints and fibers and optoelectronic systems such as optical waveguides, Bragg reflectors and dielectric mirrors.4
Electrochromic molecules which change color when electrochemically oxidized or reduced are also known in the prior art.5 For example, electrochromic systems are used in optical display and optical shutter technology and are useful as variable-transmission filters. Electrochromic displays (ECDs) are potentially superior to cathode ray tube (CRT) and liquid crystal displays (LCDs) since they consume comparatively little power, exhibit display memory effects (i.e. persistence of an image after power is removed), and provide greater opportunities for varying image tone by applying a greater electrical charge. ECDs are also very flexible since the alignment of layers in a multi-layer device is not as critical. Composite electrochromic systems providing more flexibility in color may be readily designed. ECDs may also potentially be more useful than CRT and LCD technology for large-area displays and transmissive light modulators, such as windows and optical shutters.
Heretofore “dual mode” compounds based on 1,2-dithienylcyclopentene skeleton that are both photochromic and electrochromic due to induced ring-closing/ring-opening reactions have not been described in the prior art.6 Such dual mode compounds would offer the opportunity to fabricate more sophisticated and versatile control systems for regulating the optical properties of products. For example, composite systems comprising multiple layers can pose particular technical challenges. If all of the layers are solely photochromic, the light energy will be filtered once the first surface layer is colored and the likelihood of light penetrating all of the interior layers is low. Moreover, an interior layer cannot be independently addressed using light alone unless the system is capable of two-photon-mode photochromism. Electrochromism provides a means to access each layer individually since a multilayer device can be constructed of individual insulated electrode films. Many other applications may envisaged where it would be convenient to reversible change the color of a product by both photochromic and electrochromic means. It would be particularly advantageous if the electrochromic trigger could be implemented in a catalytic electrochemical process to minimize the required energy input.
The need to incorporate photochromic and electrochromic molecules into workable materials such as films, sheets, fibers or beads demands them to be in polymeric rather than monomeric forms.7 Ring-Opening Methathesis Polymerization (ROMP) is an ideal method for synthesizing functional polymers with narrow molecular weight distributions due to the mild reaction conditions needed and its compatibility with a wide range of functional groups.8 In addition, the polymer chain length can be readily tailored by varying the catalyst/monomer ratio. The versatility of ROMP for generating photochromic polymers, including polymers having a variety of pendant functional groups, is described in the Applicant's PCT application No. PCT/CA01/01033 (WO 02/06361) which is hereby incorporated by reference. As described in the '033 application, homopolymers (i.e. polymers derived from one species of monomer) are more desirable than copolymers as they will have an increased density of the photochromic unit within the material.9 This translates into a greater amount of information expressed or stored per unit volume or surface. While the photochromic homopolymers described in the '033 application are very useful, the density of the homopolymers is limited by the fact that the active photochromic component is located on a side chain of the polymer. In order to create ultra-high density homopolymers it would be desirable if the active component could be arranged directly on the main-chain or backbone of the polymer.
A need has therefore arisen for dual mode compounds having physical properties which may be controlled be controlled both photochemically and electrochemically and improved homopolymers having a more dense arrangement of active chromic components, both in solution and in solid-state forms.